The invention relates to marine propellers, and more particularly to flow through type propellers.
Flow through type marine propellers discharge engine exhaust gases through the propeller and beneath the water level at a location behind the boat. Propellers of this type include an inner hub, which is driven by the propeller shaft, and an outer hub which is spaced radially outwardly of the inner hub and is connected thereto by a series of radial spokes legs. The space between the hubs defines an exhaust passage through which the engine exhaust gas is discharged beneath the water level. A plurality of blades are formed integrally with the outer hub.
To improve engine and boat performance, it is known in the prior art to provide a diffuser ring at the trailing end of the outer hub. In Cleary U.S. Pat. No. 5,158,433, incorporated herein by reference, the outer hub has an integral aft skirt portion flared outwardly to a trailing end to provide a diffuser ring. The diffuser ring assists in exhaust gas flow and provides a pressure barrier that helps prevent exhaust gases from feeding back into the propeller blades. This enhances engine and boat performance.
In one aspect of the present invention, structural design improvements are provided enhancing the forming operation of the flare, and reducing cracking and fatigue.
In another aspect of the invention, structural design improvements are provided further enhancing engine and boat performance by preventing exhaust in the exhaust passage from seeking the negative pressure backside surfaces of the propeller blades.